checkFile method
Checks a file
with a manifest file.
Implementation
Future<bool> checkFile(File file) async {
if (!file.existsSync()) return false;
var length = file.lengthSync();
if (length != this.length) return false;
var data = file.readAsBytesSync();
var dataSHA256 = data.computeSHA256();
return sha256.equals(dataSHA256);
}